Osmosis

Osmosis. Assemble the setup shown in Fig. 48. Cover the wide -opening of a glass vessel that is a bottomless jar with an animal blad- 

How does the osmotic pressure depend on the temperature and the solute concentration State van t Hoff s law. 

Osmosis is a process that allows the small solvent molecules, like water, to pass through a semi-permeable membrane but prevents the larger solute, such as sugar molecules, from passing through. Solvent passes from a more dilute solution (rich in solvent) through the semi-permeable membrane into the more concentrated solution (rich in solute). 

Osmosis is an important process in any biological system, i.e. plants and animals, including ourselves. Many of the cell processes depend upon the ability of cell walls to act as semi-permeable membranes and allow the passage of fluids depending upon the concentrations of solutions inside and outside the cells. 

The numerical value of the osmotic pressure depends upon the total concentration of the solute and thus the numbers of particles or ions present. When two solutions 

Reverse osmosis is a demineralization process that relies on a semi-permeable membrane to effect the separation of dissolved solids ftom a liquid. The semipermeable membrane allows liquid and some ions to pass, but retains the bulk of the dissolved solids. Although many liquids (solvents) may be used, the primary application of RO is water-based systems. Hence, all subsequent discussion and examples will be bas on the use of water as e liquid solvent. 

To understand how RO works, it is first necessary to understand the natural process of osmosis. This chapter covers the fundamentals of osmosis and reverse osmosis. 

Osmosis is a natural process where water flows through a semipermeable membrane from a solution with a low concentration of dissolved solids to a solution with a high concentration of dissolved solids. 

The difference in height between the 2 compartments corresponds to the osmotic pressure of the solution that is now at equilibrium. 

Osmotic pressure (typically represented by Jt (pi)) is a function of the concentration of dissolved solids. It ranges from 0.6 to 1.1 psi for every 100 ppm total dissolved solids (TDS). For example, brackish water at 1,500 ppm TDS would have an osmotic pressure of about 15 psi. Seawater, at 35,000 ppm TDS, would have an osmotic pressure of about 350 psi. 

Due to the added resistance of the membrane, the applied pressures required to achieve reverse osmosis are significantly higher than the osmotic pressiu-e. For example, for 1,500 ppm TDS brackish water, RO operating pressures can range from about 150 psi to 400 psi. For seawater at 35,000 ppm TDS, RO operating pressures as high as 1,500 psi may be required. 

The phenomenon of osmosis is another example in which molecules flow from a location of higher chemical potential to a region of lower potential. In an osmotic experiment a sample of dispersion in compartment II is separated from pure medium in compartment I by a semi-pcrmcable membrane which allows passage of molecules of the medium but is completely imperme- 

The increase in chemical potential of the solvent when the applied pressure is increased by dp is dfii = vtdp, so that 

This equation can be simplified if v i is close to unity (dilute dispersion) since then 

When the mass concentration of particles, p = m, /]. replaces the mole fraction, i.e  

In principle, therefore, the molar mass of the particles can be calculated from measurements of II as a function of p in a dilute suspension. In practice, the magnitude of II is so small for particulate dispersions (e.g. for c2 = 101 particles cm 3, n = 4 X 10 5 atm) that osmometry does not provide a useful route to particle size determination. It is nevertheless used for the determination of the molar mass of macromolecules, although 

Richard G., Reverse Osmosis, in Handbook of Industrial Membrane Technology, M.C. Porter, Ed., William Andrew Publishing, 1990. 

Cadotte, John, R.S. King, R.J. Majerle, and R.J. Peterson, Interfacial Synthesis in the Preparation of Reverse Osmosis Membranes, Journal of Macromolecular Science and Chemistry, A15,1981. 

Glater, Julius, Professor Emeritus, UCLA, personal communications, February 24,2009. 

Advanced Membrane Technology and Applications, Li, Norman, Anthony Fane, W.S. Winston Ho, and Takeshi Matsuura, eds., John Wiley Sons, Inc., Hoboken, NJ, 2008. 

Murphy, I. Wiater-Protas, and H.F. Ridgway, Development of a New Chlorine and Biofouling Resistant Polyamide Membrane, technical report number A273214 under theSBIR contract number DAAD19-02-C-0031. 

Diffusion of solvent through the ion exchange membrane is called osmosis, which is caused by a difference in the chemical potential of the solvent across the membrane. Solvent is transported from the dilute solution to the concentrated and osmotic water is proportional to the osmotic pressure between the two solutions, II, which is expressed as follows (aw and 5W activities of solvent in the solution and in the membrane phase), 

The electrolyte flux is naturally affected by osmosis. Namely, a strong positive osmosis carries the electrolyte from the dilute solution to the concentrated one, which is incongruous salt flux. Conversely, electrolyte diffusion is retarded when the mobility of the co-ion is faster (negative osmosis). The flux of the solvent provides the energy required to transfer the electrolyte against its chemical potential gradient. 

Anomalous osmosis is apt to occur in a charged membrane with low resistance to solvent flow, a high concentration of fixed charge and in a solution of electrolyte for which the counter-ions and co-ions have a large difference in mobility, such as acids and alkalis. 

The presence of solutes in water can have profound effects upon the properties of the solvent. These effects include lowering the freezing point, elevating the boiling point, and osmosis. All such properties are called colligative properties they depend upon the concentration of solute, rather than its particular identity. The effects of solutes and solution concentrations on colligative properties are addressed briefly here. 

Human blood consists chiefly of red blood cells suspended in a fairly concentrated solution or plasma. The dissolved material making up the solution is mostly sodium chloride at around 0.15 molar concentration. The red blood cells also contain a solution much like the plasma in which they float. If one were to take some blood. 

Many biological membranes, such as the wall surrounding a red blood cell, allow material to pass through. Water in the ground tends to move through cell membranes in root cells and finally to plant leaves, where it evaporates to form water vapor in the atmosphere (a process called transpiration). Osmosis is a major part of the driving force behind these processes. 

Cell containing H2O and a iess concentrated sail soiution 

Osmosis may result in the buildup of a very high pressure called osmotic pressure. It is this pressure that can get so high in a red blood cell suspended in water that the cell bursts. Osmotic pressure can be several times atmospheric pressure. 

In many technical applications, liquids with different concentrations are separated by so-called semipermeable membranes, this means membranes which are permeable only for the solvent but not for the dissolved species. Phase equilibrium between these two liquids can only be achieved by diffusion of the solvent through this membrane. This happens in a way that the solvent is transported from the solution with the lower solute concentration to the solution with the higher concentration. This phenomenon is called osmosis. 

Completely semipermeable membranes do hardly exist. Most of the membranes used in practical applications can generally be passed by substances with a low molar mass, whereas large molecules are rejected. In most of the cases, the solvent is water, an example for a membrane is cellophane. 

The equilibrium between the solutions is specified by equal temperatures and fugacities in the two phases, whereas the pressures will be different as shown below. 

Chemical Thermodynamics for Process Simulation, First Edition. 

Jurgen Gmehling. Barbel Kolbe, Michael Kleiber, and Jurgen Rarey. 

To understand why cells neither collapse nor burst easily, we need to explore the 

The phenomenon of osmosis is the passage of a pure solvent into a solution separated from it by a semipermeable membrane, a membrane that is permeable to the solvent but not to the solute (Fig. 3.34). The membrane might have microscopic holes that are large enough to allow water molecules to pass through, but not ions or carbohydrate molecules with their bulky coating of hydrating water molecules. The osmotic pressure, 17 (uppercase pi), is the pressure that must be applied to the solution to stop the inward flow of solvent. 

In the simple arrangement shown in Fig. 3.34, the pressure opposing the passage of solvent into the solution arises from the hydrostatic pressure of the column of solution that the osmosis itself produces. This column is formed when the pure solvent flows through the membrane into the solution and pushes the column of solution higher up the tube. Equilibrium is reached when the downward pressure exerted by the column of solution is equal to the upward osmotic pressure. A complication of this arrangement is that the entry of solvent into the solution results in dilution of the latter, so it is more difficult to treat mathematically than an arrangement in which an externally applied pressure opposes any flow of solvent into the solution. 

The osmotic pressure of a solution is proportional to the concentration of solute. In fact, we show in the following Justification that the expression for the osmotic pressure of an ideal solution, which is called the van t Hoff equation, bears an uncanny resemblance to the expression for the pressure of a perfect gas  

Because n /V = [B], the molar concentration of the solute, a simpler form of this equation is 

Blood and lymph are approximately isotonic to a cell so that cells do not gain or lose liquid when bathed in these fluids. Pure water is hypotonic and may cause cells to swell and burst. During intravenous feeding, injections, and storage of cell tissue, a salt (saline) solution is used with a concentration of solutes that is essentially isotonic with blood (and hence, with the cell) to prevent cell damage. 

1 For mixtures of 1-hexene (component l) + n-hexane (component 2), the total vapor pressure p above the mixture is related to Ay and ty, the mole fraction in the liquid and vapor phases as follows  

2 The following table gives the partial molal volumes at T = 298.15 K of ethyl acetate (1) and carbon tetrachloride (2) in solutions of the two. 

4 At T = 298.15 K, the excess molar enthalpies and the excess molar Gibbs free energies G for (.V] c-CeH +. v c-C6HnCH3) are 

Make a graph of 7, // , and TS against a, and compare the values. E7.5 Use the Debye-Hiickel theory to calculate the activity at 298.15 K of CaCl2 in the following aqueous solutions  

The plasma membrane is semipermeable because it is not permeable to all solute particles present. As a result, it maintains a concentration difference for many ions and molecules across itself, although water crosses the membrane freely in either direction. The movement of water in and out of the 

Intravenous (i.v.) solutions are commonly administered to patients in hospitals, long-term care facilities, and ambulances. They are used primarily to replace body fluids and to serve as a vehicle for injecting drugs into the body. The advantages of this pharmaceutical dosage form include the rapid onset of action, the ability to treat patients unable to take medication orally and the ability to administer a medication unavailable in any other dosage form. 

Intravenous solutions must be isosmotic (same osmotic pressure) with red blood cells. If red blood cells were to be exposed to an i.v. solution that was hypoosmotic (lower osmotic pressure), water would move into the cells causing them to swell and possibly lyse. If red blood cells were to be exposed to a hyperosmotic i.v. solution (higher osmotic pressure), water would move out of the cells causing them to dehydrate and shrink. Both of these conditions would damage the red blood cells and disrupt function. 

Patient discomfort is another important consideration. The stinging caused by a hypoosmotic or hyperosmotic i.v. solution is not experienced with one that is isosmotic. Intravenous injections are often prepared with 0.9% sodium chloride or 5% dextrose, both of which are approximately isosmotic with red blood cells. 

When red blood cells are put into water they burst. What is happening The cell membrane allows only water to pass through. The fluid in the red cells contains a mixture of organic compounds and inorganic ions, and is therefore more concentrated than the water outside. Water enters through the cell walls in an attempt to dilute the solution within the cell. Eventually, so much water enters that the cell wall bursts apart. 

If fresh red blood cells are placed in a 2% solution of salt, the cells shrink. In this case, the water within the cells enters the more concentrated solution outside the cell, and the cells dehydrate. 

Osmosis is the passage of a solvent (usually water) from a zone of low concentration to one of high concentration. The solution of higher concentration is said to be hypertonic ( hyper- means more), whereas the solution of lower concentration is said to be hypotonic ( hypo- means less). After osmosis is complete, the solutions are equally concentrated and are said to be isotonic ( iso- means the same). An example of osmosis is shown in Fig. 11.10. 

The membrane allows solvent to pass through but not solute it is called a semipermeable membrane. 

The movement of solvent from low to high concentration is similar to the spreading out of a gas (diffusion) from high gas pressure to low pressure. The idea of pressure is also applicable to osmosis, and the osmotic pressure may be thought of as the force acting per unit area of membrane which drives solvent molecules through the membrane. 

Natural ability without education has more often raised man to glory than education without natural 

Related to the diffusion of materials is the process of osmosis, which occurs mainly across semi-permeable membranes found in living things. Manbranes are used to separate different molecules and to control local environmental conditions. There are manbranes surrounding the cell and many of its inclusions. 

FIGURE 2.8.3 Placing a semipermeable membrane between an aqueous solution and pure water results in a movement of water from the pure water side to the salt solution side that is halted when the difference in heights of the two liquids counterbalances the osmotic pressure. 

If there is a higher concentration of a solute on one side of the membrane, there is consequently a lower concoitration of water (as long as total pressures mi both sides of the membrane are approximately equal). In other words, if the mass fraction of solute is higher, the mass fraction of water must be lower. 

Osmosis is important in living things becanse it helps to maintain water balance, and, sometimes when solute concentrations are too high either inside the cell or outside, can cause cellular death. 

Another good example of the first effect would be salted slices of white radish (Experiment 12.6). 

Experiment 12.6 Juice extraction from slices / salted white radish The radish is cut into thin slices and these slices are piled in two stacks. The slices of one of the stacks are picked up in turn, salted very well, and piled up again. Subsequently, both stacks are speared on the wire. Immediately, juice begins to drip out of the stack with the salted slices. The measuring cylinder contains approx. 30 mL Juice after 15 min. 

Experiment 12.7 Swelling of a decalcified egg in water One of two raw eggs (as equal as possible in size) is placed in a beaker with hydrochloric acid (or vinegar) to dissolve the calcareous egg shell— without breaking the membrane surrounding the egg. Subsequently, each of the eggs is put in a separate beaker filled with water. After 2 days, the shell-less egg has grown visibly in size. 

The second effect can also be demonstrated on a raw decalcified egg which is cautiously placed in water (Experiment 12.7). 

A gradual excess pressure results from the flow of solvent A into the concentrated solution. The chemical potential y A so gradually increases so that the potential gradient decreases. The process stops when on the right and on the left of the wall becomes equal (or when the substance A completely disappears from one side). The resulting excess pressure is called osmotic pressure. 

Certain materials, including many membranes in biological systems and synthetic substances such as cellophane, are semipermeable. When in contact with a solution, these materials allow only small molecules—water molecules, for instance— to pass through their network of tiny pores. 

Consider a situation in which only solvent molecules are able to pass through a semipermeable membrane placed between two solutions of different concentrations. The rate at which the solvent passes from the less concentrated solution (lower solute concentration but higher solvent concentration) to the more concentrated solution (higher solute concentration but lower solvent concentration) is greater than the rate in the opposite direction. Thus, there is a net movement of solvent molecules from the solution with a lower solute concentration into the one with a higher solute concentration. In this process, called osmosis, the net movement of solvent is always toward the solution with the higher solute concentration, as if the solutions were driven to attain equal concentrations. 

If the pure water in the left arm of the U-tube is replaced by a solution more concentrated than the one in the right arm, what will happen  

Net movement of H2O is from pure water side to solution side 

The osmotic pressure obeys a law similar in form to the ideal-gas law, UV = nRT, where 77 is the osmotic pressure, V is the volume of the solution, n is the number of moles of solute, R is the ideal-gas constant, and T is the Kelvin temperature. From this equation, we can write 

At equilibrium, the flow of H2O is the same in both directions, so there is no net movement of H2O 

In many circumstances polymers may act as a semi-permeable membrane when exposed to a mixture of external vapor or fluid molecules. This phenomenon results in an osmotic pressure that induces internal damage and affects the diffusion process, rendering it sensitive to the detailed composition of such mixture (Ashbee 1989). 

For many vapors and polymeric composites the aforementioned quantities were related empirically by the expression 

Where and are constants, and RH is in %. Other forms are also possible. 

---

Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

Reverse osmosis is used for desalination of seawater, treatment of recycle water in chemical plants and separation of industrial wastes. More recently the technique has been applied to concentration and dehydrogenation of food products such as milk and fruit juices. See ultrafiltralion. 

Related phenomena are electro-osmosis, where a liquid flows past a surface under the influence of an electric field and the reverse effect, the streaming potential due to the flow of a liquid past a charged surface. 

---